This invention relates to the measurement of a quantity, linear heat generation rate, often abbreviated LHGR. The word linear is used because the quantity is a measure of the heat generated within one unit length of the fuel pin of a nuclear reactor. The LHGR of a fuel pin will be different at different locations along its length since fission depends, in part, on proximity of other sources of radioactivity.
The quantity is important since, if the quantity goes too high, the cladding of the fuel pins is in danger of melting. Thus, the linear heat generation rate is as important to safe nuclear reactor operation as is reliability of coolant flow.
The gamma thermometer array of the above-identified Rolstad et al. application contains a plurality of thermocouples which operate at individual temperatures which are multivariable functions of the local heat generation rate and the individual thermal resistance between the thermocouples and their respective heat sinks, cooled by the circulating reactor coolant. The said individual thermal resistance to the respective heat sink, whether the reactor be boiling or pressurized, is accurately known, and thus the local heat generating rates can be calculated from the readings of the plural thermocouples.
The calculation of local heat generating rate from the data furnished by the gamma thermometer array of Rolstad et al. is fairly straightforward and simple compared to the corresponding calculation required to determine the same quantity using data furnished by the more common miniature fission chambers and self-powered neutron detectors, which respond almost entirely to thermal neutrons. The latter calculations must take into account the variability among the individual sensors, and must also be corrected for the continuous depletion of emitter material in the sensor.
Furthermore, the local thermal neutron flux, as calculated from the corrected output of miniature fission chambers and self-powered neutron detectors, must be converted into local heat generation rate by a complex calculation which takes into account the fact that, as the U.sup.235 of the reactor is being depleted by the fission process, the local heat generating rate goes down, but the local thermal neutron flux goes up. Thus, the quantity being directly measured and the calculated quantity of interest are inversely and very complexly related. The situation often arises that the magnitude of the corrections to the basic signal is three times larger than that of the basic signal itself.
A problem with neutron sensors is that they cannot be manufactured identically. Furthermore, they cannot be calibrated reliably before being placed in service, because there is no source of sufficient neutron flux, outside of a reactor, to calibrate them. In the event such source were available, the radioactivity induced by calibration would render further handling economically impracticable.
In contrast, and uniquely for such instruments, gamma thermometers can be manufactured nearly identically, and can be tested at the point of manufacture to prove the relationship of each signal output to the heat generated in the sensors. Their signals can thus be relied upon to reflect accurately the heat being generated within the sensors and data, available in the open literature, establish that the heat to signal relationship is constant for many years.
There does remain, however, the problem of relating the measured heat generation rate in the sensor to the unknown LHGR of adjacent fuel pins which the sensors purport to measure.
The single available truly independent measurement against which this property can be tested in any light water reactor is the average fuel LHGR, which is determined by calorimetry. To determine average fuel LHGR the total heat being generated in the core is calculated by combining measurements of total mass flow and temperature rise of the coolant and small corrections made for heat not being generated in the fuel pins themselves. By dividing the total heat rate so obtained by the total length of fuel in the core an average value of LHGR is obtained.
The distributed in-core sensors, however, purport to measure local LHGR, not average heat rate. If a reactor contained a large number of such sensors uniformly distributed, their signals could be averaged and compared to, and uniformly corrected if necessary, for changes in the ratio of average signal to average LHGR, which changes might take place over the several year lifetime of the fuel assemblies.
In accordance with the instant invention, a traveling miniature gamma thermometer is provided, and is caused to travel through bores which are distributed in an array through the reactor core.
In some cases the traveling gamma thermometer would have to be on a miniature scale, compared to gamma thermometers of the prior art, in order to be able to fit, for example, in a dry bore located in the inner rod of the gamma thermometer of the Rolstad application. In other cases diameters up to 1/4 or 3/8 inch might be allowable when used in the in-core instrument thimbles employed in many reactors.
The traveling gamma thermometer need not be precisely calibrated during manufacture, since its inherent stability permits it to correctly reflect differing relative levels of activity as it is traversed through the nuclear reactor core, unless extremely poor thermal contact between the traveling gamma thermometer and the associated bore should cause the unit to be many degrees hotter; on the order of 100.degree. C., than its nominal value.
This stability, during the core scanning, permits the construction of an accurate three dimensional relative gamma flux activity level plot of the reactor core. With the use of calorimetry, the relative gamma levels are readily changed to LHGR levels by determining the ratio between average gamma ray activity and average LHGR and correcting local readings by this ratio.
Thus, the minature traveling gamma thermometer of this invention is adapted to scan the cross sections of a reactor core to determine the levels of activity at different locations in the core, and the results of such scanning can be used to determine LHGRs within the core and, if the reactor is so equipped, to verify theoretical calibrations of the in-place gamma thermometers of a full or partial gamma thermometer array of the Rolstad et al type, using methods of calorimetry.
If a full and symmetrically related system of gamma thermometers of the Rolstad type were in place in the reactor core then the average of all of the measurements could be related to the calorimetrically determined average LHGR of the fuel and an experimental ratio determined between LHGR and gamma thermometer signal to verify the theoretical value of this ratio. In this case the system would gain little or nothing in calibration precision through use of the traveling gamma thermometer of the instant application. For partial, non-symmetric or sparse systems of the Rolstad type of gamma thermometers, however, a calibration of average LHGR indicated against average LHGR from calorimetry can be insufficiently precise and calibration precision could be greatly increased by use of a traveling gamma thermometer of the instant application.
The traveling gamma thermometer of the instant application finds an important application in precision calibration of full systems of older types of unstable neutron measuring instruments such as fission chambers or self-powered neutron detectors which have not yet been or cannot easily be replaced by fixed gamma thermometer systems of the Rolstad type. In fact, traveling probe systems employing either fission chambers or self-powered neutron detectors have been found necessary in pressurized water reactors of two major types after the stability of these fixed neutron instruments proved unsuitable and have always been necessary to frequently recalibrate the very unstable fixed instruments used in boiling water reactors. For reactors employing such unstable fixed systems and for reactors using only traveling probes the traveling gamma thermometer of the instant application provides a superior method of local LHGR determination.
The traveling gamma thermometer described herein is useful in a proof-of-precision application to demonstrate the precision of the Rolstad type of gamma thermometer before a full system is in place. For example, if only three or four Rolstad units were in place in a reactor whose full, symmetrical compliment was 50 units from each containing 7 sensors, then the LHGR's calculated from these could be shown to be identical over a long period of time with LHGRs from a traveling gamma thermometer which had traversed the entire core and had been repreatedly consistent against overall calorimetric calibration as described above.